1. Field of the Invention
The present invention relates to a fixing apparatus that is applied in an image forming apparatus using an electrophotographic method, such as a copying machine, a facsimile, or a printer.
2. Related Art
In an image forming apparatus of this type using an electrophotographic method, an electrostatic latent image is formed on the surface of a photosensitive drum. Then, the electrostatic latent image on the surface of the photosensitive drum is developed using toner, and a toner image is thereby formed on the surface of the photosensitive drum. Thereafter, the toner image is transferred from the photosensitive drum onto recording paper, the recording paper is heated and pressed, and then the toner image is fixed onto the recording paper.
A fixing apparatus fixes a toner image onto recording paper. In the fixing apparatus, recording paper is sandwiched in a nip region between a hot roller and a pressure roller, which form a pair, and is transported. Accordingly, the recording paper is heated and pressed by the rollers, so that toner on the recording paper is heated and melted so as to be fixed.
In the fixing apparatus as described above, in order to retain the width of the nip region between the hot roller and the pressure roller and stably maintain the nip region, by covering the rollers with an elastic layer, the elastic layers of each roller are pressed against each other and deformed.
However, since the pressure between the rollers is large, when the fixing apparatus is not used and the rollers are in a stopped state for a long time, the elastic layers of the rollers are left deformed and sometimes unable to return to their original shape, thus forming depressions in the elastic layers of the rollers.
Consequently, for example, JP 2003-280308A discloses a configuration in which a hot roller and a pressure roller are separated when the rollers are stopped so as to prevent the elastic layers of the rollers from being left deformed. In that configuration, a frame body that axially supports a pressure roller is movably supported, a cam that slidingly contacts this frame body is provided, the frame body is moved by this cam and, thereby, the pressure roller can be separated from the hot roller. When a fixing operation is performed, the forward rotation of an output shaft of a driving motor is transmitted to the hot roller via a gear unit and a one-way clutch, and then recording paper is sandwiched in the nip region between the rollers and transported. Also, when a fixing operation is not performed, backward rotation of the output shaft of the driving motor is transmitted to the cam via another gear unit and another one-way clutch, the frame body is moved by the cam, and then the pressure roller is separated from the hot roller. As a result of this, it is possible to switch and perform rotational driving of the rollers and an operation to bring the rollers into and out of contact with each other by using one driving motor.
Meanwhile, a driving motor of a fixing apparatus is not always used only for the fixing apparatus, and is often used for driving the rotation of the rollers for transporting recording paper on an upstream side and downstream side in the recording paper transport direction, or the rollers for developing and transferring an image, as well. That is, one driving motor drives the rotation of various rollers.
Accordingly, when the output shaft of the driving motor is rotated backward, various rollers also rotate backward, which causes various problems.
As disclosed in the aforementioned JP 2003-280308A, if some one-way clutches are mounted in the gear unit, it is possible to avoid the backward rotation of various rollers and the like even when the output shaft of the driving motor is rotated backward. However, since one-way clutches are mounted in the gear unit itself, the configuration of the gear unit becomes complex.